A demand for faster and more accurate chemical analyses has motivated development of several analytical techniques. Two such techniques are Liquid Chromatography (LC) and Mass Spectrometry (MS).
FIG. 1 is an example of a typical LC chromatogram obtained through use of an Ultraviolet (UV) absorption detector. The chromatogram includes several peaks associated with different sample compounds. The duration of a LC analysis, also known as a run, typically requires a few minutes to a few hours or more, depending on the type of sample. Most LC detectors provide an item of information, such as an intensity value, per selected unit of time. The collected information provides a two-dimensional (2-D) graph—a chromatogram—of detector-measured intensity versus retention time. Peaks in the graph indicate the presence of separated sample compounds.
In contrast to LC, MS provides mass-related information for sample compounds. Prior to mass analysis, sample compounds are ionized and often fragmented. A MS instrument uses electric and/or magnetic fields to accelerate ions and direct them to an ion detector. In some MS instruments, fields are swept to cause mass-analyzed ions having a range of mass-to-charge ratio (m/z) values to reach the ion detector at different times. Completion of a mass scan typically requires less than a second, and covers a wide range of m/z values, typically from 50 Atomic Mass Units (AMU) per charge to about 2000 AMU per charge.
In Time-of-Flight (TOF) MS instruments a collection of ions of different m/z are simultaneously directed to an ion detector. Ions having a smaller m/z value reach the detector before those having a larger m/z value.
A typical ion detector provides an ion intensity response that is proportional to the number of ions that strike it at any given time. Thus, raw intensity data obtained from the detector is proportional to m/z values rather than to the mass of the associated ions. Often, however, the mass spectrometer's output data is simply referred to as mass data.
FIG. 2 is an example of a graph of ion intensity versus m/z, which illustrates a typical graph of data derived via MS. The spectrometer generates an array of information points having intensity values that vary with mass value, that is, with m/z value. The 2-D graph of this data is known as a scan. The peaks of a MS scan are typically sharper and more abundant than those observed in a LC chromatogram.
Though LC and MS are often used independently, some instruments combine these techniques such that the eluent of an LC column is utilized as a sample source for a MS. This compound technique, known as LC/MS, exploits both the compound separation capabilities of LC and the detection sensitivity of MS.
In LC/MS, MS produces a mass scan per unit of retention time of the LC device. The LC/MS instrument thus produces ion-intensity values associated with corresponding ion-m/z values and with chromatographic retention-time values. Because a mass scan may contain a few thousands to almost half a million data points, or more, and a chromatogram may have a few hundred to a few thousand data points, one notes that LC/MS analyses easily generate huge amounts of data, even for a single sample. Thus, a full data set obtained from a single sample often includes millions or billions of data points each associated with chromatographic retention time, m/z, and ion intensity.
For example, a 2000 AMU scan that has ten data points per 0.05 AMU to resolve a 0.05 AMU minimum peak width provides 400,000 data points per mass scan. If one scan per second is obtained during a two-hour LC run and each data point is represented by a 32-bit floating point number, the uncompressed LC/MS data set will include over ten gigabytes of computer-related data. Evaluation of such enormous quantities of data presents a significant challenge to chemical analysts.